(a) Field of the Invention
The invention relates to a dry etching method and apparatus, and more particularly, it relates to an improved dry etching method and apparatus which utilize gaseous plasma and provides high precision etching.
(b) Description of the Prior Art
Along with the advances and developments made of late in techniques in the integrated circuit designing field, higher precision is required in the manufacture of integrated circuits without being entailed by an increase in the manufactureing cost and yet allowing mass-production to be performed with no difficulty. Furthermore, the prevention and avoidance of pollution has been constituting a big problem that requires to be solved. Under such circumstances, dry etching is attracting the attention of those concerned in view of its low-pollution and high precision properties.
The so-called dry etching techniques may be divided roughly into the following two types, i.e. the ion beam etching technique and the plasma etching technique. In the ion beam etching technique, an ion beam (including electron beam) is irradiated onto a specimen (target) in a high vacuum (usually the ambient pressure is less than 10.sup.-5 Torr) and physically sputters away the irradiated surface atoms. In case the radius of the ion beam is reduced by electronic lens system, a maskless etching can be performed by sweeping the ion beam on the specimen. In case positive ions are used as the etching agent, surface charge-up of the specimen can be prevented by neutralizing the accelerated or bombarded positive ions with electrons. Furthermore, ion beam etching allows accurate and directional etching to be achieved, since the etching agent collides with the target in one direction. However, the total etching rate in the case of the ion beam etching is rather low due to the fact that the diameter of the beam is small, and accordingly it is not possible to accomplish etching of a wide area easily. In addition, the surface which is subjected to sputtering be accelerated ion beam tends to become rough, and thus a damaged surface could be produced as a result of this etching. Therefore, the grade of finish of the resulting processed surface will low from the viewpoint of the crystallographic and electronic properties. If the acceleration voltage is lowered in order to reduce the surface damage, the etching rate will decrease further.
On the other hand, plasma etching is being developed with a great rapidity. In this plasma etching technique, a specimen carrying a mask thereon is placed in a radio frequency plasma and is subjected to etching. Since an alternating electric field is used, the charge-up of the surface is neutralized in each cycle. Plasma gas, in most cased, is chemically reactive to the material to be etched. For example, freon (CF.sub.4) gas plasma may be used for etching silicon and silicon oxides, and oxygen (O.sub.2) gas plasma may be used for etching photoresist films. The chemical reaction mechanisms have been debated but no decisive conclusion has been reached yet. Also, the role of neutral radical atoms in the reactive plasma etching is still in the stage of discussion.
Reactive plasma etching is regarded generally as a dry chemical reaction, and the optimum pressure of the plasma gas employed is usually of the order of 10.sup.-1 to 10.sup.0 Torr. At such a pressure, the mean free path is still very short, and electrons, ions, atoms (including radicals) and molecules (hereinafter they are generally referred to as component particles) repeat collisions and move randomly according to the Maxwell-Boltzmann distribution. Accordingly, component particles which are effective for etching cannot have high kinetic energy. As can be expected, such reactive plasma etching is almost isotropic in direction due to the random motion of the component particles. Thus, side etching does occur similar to the wet etching. Furthermore, the released atoms or molecules in the plasma etching technique may redeposit on the specimen. If the pressure of the gas plasma is reduced for solving these problems, the etching rate will also be reduced. Furthermore, since the degree of vacuum (not pressure) employed for reactive plasma etching is considerably low (of the order of 10.sup.-1 to 10.sup.0 Torr), the effect of residual gas is not negligible. Also, even when ions of a certain polarity are effective for etching, and if those of the opposite polarity are harmful for some reasons or other, there indeed no way of preventing the reaction on the speciman by those ions having said opposite polarity, while utilizing those ions having said certain polarity required for etching.